Ras-like proteins, particularly members of the Rab GTPase subfamilies, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of this subfamily of Ras-like proteins. The present invention advances the state of the art by providing a previously unidentified human Ras-like proteins that have homology to members of the Rab GTPase subfamilies.
Ras Protein
Ras proteins are small regulatory GTP-binding proteins, or small G proteins, which belong to the Ras protein superfamily. They are monomeric GTPases, but their GTPase activity is very slow (less than one GTP molecule per minute).
Ras proteins are key relays in the signal-transducing cascade induced by the binding of a ligand to specific receptors such as receptor tyrosine kinases (RTKs), since they trigger the MAP kinase cascade. The ligand can be a growth factor (epidermal growth factor (EGF), platelet-derived growth factor (PDGF), insulin, an interleukin (IL), granulocyte colony-stimulating factor (G-CSF), granulocyte/macrophage colony-stimulating factor (GM-CSF).
Ras proteins contain sequences highly conserved during evolution. Their tertiary structure includes ten loops connecting six strands of beta-sheet and five alpha helices.
In mammalians, there are four Ras proteins, which are encoded by Ha-ras, N-ras, Ki-rasA and Ki-rasB genes. They are composed of about 170 residues and have a relative molecular mass of 21 kD. Ras proteins contain covalently attached modified lipids allowing these proteins to bind to the plasma membrane. Ha-Ras has a C-terminal farnesyl group, a C-terminal palmitoyl group and a N-terminal myristoyl group. In Ki-Ras(B), a C-terminal polylysine domain replaces the palmitoyl group.
Ras proteins alternate between an inactive form bound to GDP and an active form bound to GTP. Their activation results from reactions induced by a guanine nucleotide-exchange factor (GEF). Their inactivation results from reactions catalyzed by a GTPase-activating protein (GAP).
When a Ras protein is activated by a GEF such as a Sos protein, the N-terminal region of a serine/threonine kinase, called “Raf protein”, can bind to Ras protein. The C-terminal region of the activated Raf thus formed binds to another protein, MEK, and phosphorylates it on both specific tyrosine and serine residues. Active MEK phosphorylates and activates, in turn, a MAP kinase (ERK1 or ERK2), which is also a serine/threonine kinase. This phosphorylation occurs on both specific tyrosine and threonine residues of MAP kinase.
MAP kinase phosphorylates many different proteins, especially nuclear transcription factors (TFs) that regulate expression of many genes during cell proliferation and differentiation.
Recent researches suggest that, in mammalians, phosphatidyl inositol 3′-kinase (PI3-kinase) might be a target of Ras protein, instead of Raf protein. In certain mutations, the translation of ras genes may produce oncogenic Ras proteins.
Ras-Like Protein
Guanine nucleotide-binding proteins (GTP-binding proteins, or G proteins) participate in a wide range of regulatory functions including metabolism, growth, differentiation, signal transduction, cytoskeletal organization, and intracellular vesicle transport and secretion. These proteins control diverse sets of regulatory pathways in response to hormones, growth factors, neuromodulators, or other signaling molecules. When these molecules bind to transmembrane receptors, signals are propagated to effector molecules by intracellular signal transducing proteins. Many of these signal-transducing proteins are members of the Ras superfamily.
The Ras superfamily is a class of low molecular weight (LMW) GTP-binding proteins that consist of 21-30 kDa polypeptides. These proteins regulate cell growth, cell cycle control, protein secretion, and intracellular vesicle interaction. In particular, the LMW GTP-binding proteins activate cellular proteins by transducing mitogenic signals involved in various cell functions in response to extracellular signals from receptors (Tavitian, A. (1995) C. R. Seances Soc. Biol. Fil. 189:7-12). During this process, the hydrolysis of GTP acts as an energy source as well as an on-off switch for the GTPase activity of the LMW GTP-binding proteins.
The Ras superfamily is comprised of five subfamilies: Ras, Rho, Ran, Rab, and ADP-ribosylation factor (ARF). Specifically, Ras genes are essential in the control of cell proliferation. Mutations in Ras genes have been associated with cancer. Rho proteins control signal transduction in the process of linking receptors of growth factors to actin polymerization that is necessary for cell division. Rab proteins control the translocation of vesicles to and from membranes for protein localization, protein processing, and secretion. Ran proteins are localized to the cell nucleus and play a key role in nuclear protein import, control of DNA synthesis, and cell-cycle progression. ARF and ARF-like proteins participate in a wide variety of cellular functions including vesicle trafficking, exocrine secretion, regulation of phospholipase activity, and endocytosis.
Despite their sequence variations, all five subfamilies of the Ras superfamily share conserved structural features. Four conserved sequence regions (motifs I-IV) have been studied in the LMW GTP-binding proteins. Motif I is the most variable but has the conserved sequence, GXXXXGK (SEQ ID NO:26). The lysine residue is essential in interacting with the .beta.- and .gamnma.-phosphates of GTP. Motif II, III, and IV contain highly conserved sequences of DTAGQ (SEQ ID NO:27), NKXD (SEQ ID NO:28), and EXSAX (SEQ ID NO:29), respectively. Specifically, Motif II regulates the binding of gamma-phosphate of GTP; Motif III regulates the binding of GTP; and Motif IV regulates the guanine base of GTP. Most of the membrane-bound LMW GTP-binding proteins generally require a carboxy terminal isoprenyl group for membrane association and biological activity. The isoprenyl group is added posttranslationally through recognition of a terminal cysteine residue alone or a terminal cysteine-aliphatic amino acid-aliphatic amino acid-any amino acid (CAAX; SEQ ID NO:30) motif. Additional membrane-binding energy is often provided by either internal palmitoylation or a carboxy terminal cluster of basic amino acids. The LMW GTP-binding proteins also have a variable effector region, located between motifs I and II, which is characterized as the interaction site for guanine nucleotide exchange factors (GEFs) or GTPase-activating proteins (GAPs). GEFs induce the release of GDP from the active form of the G protein, whereas GAPs interact with the inactive form by stimulating the GTPase activity of the G protein.
The ARF subfamily has at least 15 distinct members encompassing both ARF and ARF-like proteins. ARF proteins identified to date exhibit high structural similarity and ADP-ribosylation enhancing activity. In contrast, several ARF-like proteins lack ADP-ribosylation enhancing activity and bind GTP differently. An example of ARF-like proteins is a rat protein, ARL184. ARL184 has been shown to have a molecular weight of 22 kDa and four functional GTP-binding sites (Icard-Liepkalns, C. et al. (1997) Eur. J. Biochem. 246: 388-393). ARLI 84 is active in both the cytosol and the Golgi apparatus and is closely associated with acetylcholine release, suggesting that ARL184 is a potential regulatory protein associated with Ca.sup.2+-dependent release of acetylcholine.
A number of Rho GTP-binding proteins have been identified in plasma membrane and cytoplasm. These include RhoA, B and C, and D, rhoG, rac 1 and 2, G25K-A and B, and TC10 (Hall, A. Ct al. (1993) Philos. Trans. R. Soc. Lond. (Biol.) 340:267-271). All Rho proteins have a CAAX (SEO ID NO:30) motif that binds a prenyl group and either a palmitoylation site or a basic amino acid-rich region, suggesting their role in membrane-associated functions. In particular, RhoD is a protein that functions in early endosome motility and distribution by inducing rearrangement of actin cytoskeleton and cell surface (Murphy, C. et al. (1996) Nature 384:427-432). During cell adhesion, the Rho proteins are essential for triggering focal complex assembly and integrin-dependent signal transduction (Hotchin, N. A. and Hall, A. (1995) J. Cell Biol. 131:1857-1865).
The Ras subfamily proteins already indicated supra are essential in transducing signals from receptor tyrosine kinases (RTKs) to a series of serine/threonine kinases which control cell growth and differentiation. Mutant Ras proteins, which bind but cannot hydrolyze GTP, are permanently activated and cause continuous cell proliferation or cancer. TC21, a Ras-like protein, is found to be highly expressed in a human teratocarcinoma cell line (Drivas, G. T. et al. (1990) Mol. Cell. Biol. 10:1793-1798). Rin and Rit are characterized as membrane-binding, Ras-like proteins without the lipid-binding CAAX (SEO ID NO:30) motif and carboxy terminal cysteine (Lee, C.-H. J. et al. (1996) J. Neurosci. 16: 6784-6794). Further, Rin is shown to localize in neurons and have calcium-dependant calmodulin-binding activity.
Rab GTPases
The novel human protein, and encoding gene, provided by the present invention is a novel protein of the Ras family and shows the greatest degree of similarity to Rab GTPases in general and Rab37 in particular.
GTPases play important roles in a wide variety of cell functions such as signal transduction, cytoskeletal organization, and membrane trafficking. Rab GTPases are particularly important for regulating cellular membrane dynamics by modulating the activity of effector proteins that then regulate vesicle trafficking. Rab37 is a Rab GTPase that is expressed in the MC-9 mast cell line and bone marrow mast cells. Rab37 shares 74% sequence identity with Rab26 and 47% sequence identity with Rab8. The Rab8 GTPase plays important roles in Golgi to plasma membrane vesicle trafficking. Studies have suggested that Rab37 plays an important role in mast cell degranulation. Thus, Rab37, as well as other novel human Rab GTPases, are valuable as potential therapeutic targets for the development of allergy treatments (Masuda et al, FEBS Lett Mar. 17, 2000; 470).
The discovery of new human Ras-like proteins and the polynucleotides that encode them satisfies a need in the art by providing new compositions that are useful in the diagnosis, prevention, and treatment of inflammation and disorders associated with cell proliferation and apoptosis.